This invention relates generally to filter press membrane electrolytic cells. More specifically, it relates to a method for determining which membrane in a multiple unit filter press membrane electrolytic cell has been structurally damaged.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type electrolytic cell or the flowing mercury cathode-type.
Comparatively recent technological advances such as the development of dimensionally stable anodes and various coating compositions, have permitted the gap between electrodes to be substantially decreased or eliminated entirely. This has dramatically increased the energy efficiency during the operation of these energy-intensive units.
The development of a hydraulically impermeable membrane has promoted the advent of filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product obviates the need for caustic purification and concentration processing. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, continual advances have been made in the development of monopolar filter press membrane cells.
The use of a hydraulically impermeable membrane, however, presents problems should the membrane become structurally damaged, such as ruptured by the passage of a sharp object therethrough. Since commercial size filter press membrane cells comprise multiple cathode and anode units separated by a membrane, there may be up to thirteen or more membranes in each electrolytic cell unit. The exact position of a structurally damaged membrane in a electrolytic cell unit employing multiple membranes is difficult to identify without taking apart the entire filter press cell.
Typically, structural damage to one or more membranes manifests itself in several symptomatic ways. Cathode current efficiency and anode current efficiency decrease when a membrane is damaged. The cathode current efficiency decreases are detectable, such as by physically measuring the weight of the caustic produced in a container vessel and then calculating the production rate of caustic or by physically measuring the flow rate with appropriate means, for example flow totalizer units. The production rate of caustic is calculated by measuring the equivalents of caustic produced per current load and is measured in grams per gram equivalent.
The decrease in anode current efficiency is detectable because of an increase in the presence of oxygen and oxychlorides, such as hypochlorite, or chlorates, in the cell gas and the spent anolyte stream (spent brine). A change in the pH of the spent anolyte stream can also be an indicator of a decrease in anode current efficiency. The increase in the presence of oxygen may be determined by gas chromatograph testing, while the increase in the presence of oxychlorides can be detected by titration. The oxygen and oxychlorides are present because the caustic crosses through the membrane at the point of structural damage in back migration and starts to electrolyze or chemically react with the bulk anolyte. This puts hydroxyl ions back into a low pH environment which, depending on the type of anodes being used, will produce either oxygen, chlorite ions or chlorate ions.
Previously, when testing such as this detects the presence of decreased cathode current efficiency or decreased anode efficiency, the exact location of the structurally damaged membrane could be determined only by trial and error. This required that the entire electrolytic cell be taken apart and the anodes and cathodes be separated individually to check each membrane visually for structural damage. The entire process, including the diagnosis of a problem by the detection of a reduction in the cathode current efficiency or anode current efficiency and the breaking apart of the cells to find the damaged membrane or membranes could well take several days and up to a week. A loss of this much operating time for an electrolytic cell unit is costly and the steps necessary to correct the problem in this manner are labor intensive.
The foregoing problems are solved in the method of determining the location of a structurally damaged membrane in a multiple unit filter press membrane electrolytic cell after cell operating conditions and monitoring have indicated the existence of a problem.